Possible faulty speed sensor?
 

I purchased a '90 K5 7 months ago and I have only put a couple of thousand miles on it. The odometer reads 165k. My problem is that when I start it no brake warning light is lit. By the time I drive 1/4 mile or so the brake warning light comes on. Then after about 20 or 30 miles the check engine light comes on. I have replaced the cruise control stick and the cruise still doesn't work, I have checked the diaphram and that seems to be ok. When I get to about 45-50 mph the transmission seems like it wants to kick in and out of overdrive. The trans is a 700r4 (stock), running 35's.
If it is the speed sensor what color wires come from it and is it located on the driverside of the transmission? Where is a good place to to buy one from, Auto Zone quoted me a price of $160.00 but I have seen them on the net starting off @ $25.00.
Thanks