Trucks don't have a fuse, but cars of this era did. The relays are know for going out because they're mounted upside down, and a leaky windshield (not that any of these trucks have that problem...
) will allow it to fill with water and rust it up. If a new high speed blower relay doesn't fix it, I have found a lot of times that it's the relay ground wire, that is just put under one of the relay mounting screws, often times won't be tight enough since taking the screws in and out of the fiberglass box takes the threads out of them. I usually lengthen the ground wire and bolt it to a solid metal ground.