ok, before you drive yourself crazy with wiring. start from the beginning instead of trying the short cut long way around. check all the lights to make sure they have the right bulb holders for what you need. if running a dual filament bulb then the pigtail in the lamp needs to have 2 wires in it and the bulb holder needs to have a ground wire coming off it that goes to a good clean dedicated ground connection. if the bulb holder is welded or rivetted to the lamp housing for a ground then check the integrity of the connections there and also ensure the lamp has a good ground or simply run a ground wire in your harness to the lamp and make a good connection inside the lamp for your dedicated ground for that lamp. star washers are commonly used under a ground connection. I like to paint them after with some zinc primer or use some connection grease after I know everything works like ti should.
battery negative. should have a good clean ground terminal connection on the battery and cable. don't rely on a connection that looks good from the outside of the terminal or assume that because the battery and termianls are new that you shouldn't need to clean them. actually take the terminal lug off the post, clean it to bare metal as well as the post, then assemble the parts. check to ensure the cable also has a good clean crimp or joint there as well, inside the terminal, and that the cable is smooth with no lumps indicating corrosion inside the cable. a lot of problems stem from bad grounds. you can have the best gold plated doodads on the positive terminal but a circuit is only as good as it's weakest link. power is like water, it will take the path of least resistance but if there is no path stuff just don't work.
the ground also needs a pathway or a connection to the body, frame and engine so if the positive side is getting gold plated doodads then the negative side should be treated the same
the rad support should have a good clean ground connection to the frame because the front lights and horn are likely grounded on the rad support unless you wire it differently. remember that the ground wire carries the load of both marker and signal circuits so size the ground wire according to the circuit needs.
the rear lights should have a good clean ground connection to the frame rather than to the lamp which is mounted to the box because that eliminates a whole lot of connection resistance between the lamp, box and frame.
the lights all need to have a good clean dedicated ground wire from each bulb holder. some manufacturers like to use a pop rivet on the painted light housing as a ground and that is usually a problem area sooner or later.
what I have done in the past is use some cheap trailer lights when doing wiring. I can clamp them on anywhere for good visibility from the cab where the wiring is being done and then when they power up it is easy to see them. run some good grounds and connections on each one so you KNOW the problem will not be from a poor light connection, even though they are temporary. do the wiring and related tests before connecting the new wiring to the final set of lights you will use on the truck and run all the lights at the same time so you know they all work together. another thing I have done before is use an audible device so I can hear the circuit when it gets powered up, that way if my head is under the dash there is less moving up and down to see what is lighting up out back/front. an old seat belt buzzer would work even.
if you rely on the ground from the battery to be bolted to a fender or inner fender to light up the headlights, horn and signals etc then you are assuming all the body parts are connecting on nice clean bare steel so no voltage drops could occur. when the old trucks get redone we usually take pride in painting everything because we have spent hours repairing the rust from an unpainted surface. then we mount stuff on the paint and don't wanna scrub off any new paint to make a ground connection or we bolt the fenders etc all together and don't even think about how it would conduct electricity afterward, we just bolt on some lights and expect them to work. usually that means a bad ground will show up soon if not right away. a great idea is to run a dedicated ground wire from every bulb socket or accy to a good clean ground to eliminate the unsightly ground screws on new paint but also to ensure each circuit has a dedicated ground that doesn't rely on how tight we cinch down the bolts on the new tail lights or whatever. I also like to use bolts with nuts rather than screws for the ground wires. I find screws tend to work loose or corrode faster than a bolt and nut would. I clean the connection spot down to bare steel, do the connection, test the lights when done and then spray paint over the bare spot or use connection grease on interior grounds where overspray would be an issue. I have also used the aviation form a gasket on these spots because it sticks like crazy and forms a corrosion free cover over the spot. keep the air out and the corrosion will stay out too.
in the dash ensure to run a dedicated ground from each accy or gauge to a common ground spot. don't rely on the dash panel to supply a good ground in these old trucks. I once rewired a burned out motorhome dash and firewall. I used a ground terminal lug under the dash with a cable that ran to the battery. it worked like a bus bar with all the grounds terminating on the single lug. no voltage drops. the volt gauge didn't blip every time the signals blinked etc.
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just as a refresher
front marker lights and rear marker lights are separate circuits on lots of systems
front and rear signals are separate circuits because the rears are also the brake lights.
rear signals get power from the flasher unit which gets fused power when the ign is on. brake light switch output also goes to the signal switch . so brake light switch gets fused power first and from there the wire runs up to the signal switch which does the circuit switching for you internally. at the signal switch the circuit divides inside the switch to go to the right or left signal/brake light. with the signals off the power goes from the brake light switch through the signal switch and comes out to each brake light bulb. when the signals turn on the signal switch stops brake light power from that side selected and uses the power coming from the signal flasher instead for that selected side. it's all done for you if it gets wired correctly. interior lights are usually wired as a fused hot at all times to the bulb and the ground wire goes to the headlight and door switches for a ground. dash lights get wired through the rheostat in the headlight switch so they can get variable voltage for dimming.
hope that helped somebody