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First I would get a test light or a volt meter and open the fuse box under the hood. With key on, check for battery voltage on both sides of the top of fuse #22 with the fuse still in the box. Should be a 20 AMP according to the online manual. (oops on my truck it is 30 AMP).
Turn the key back off then pull the diode #12 under the hood. Turn the key back on and one pin should have battery voltage, the other one not.
If that checks good, then turn the key off and pull relay #1. See pic below.
On my truck, under relay #1 there is 12.xx volts on the pins at the 2 yellow arrows, and 5.xx volts on the pin at blue arrow. There was 0 volts on the other pin.
Turn the key back on and check for voltage on the pins under the relay.
If that checks ok, turn the key back off and swap relay #1 and relay #3 (horn relay) and see if that makes a difference.
If you have already done this then I am sorry to waste your time.
Did the WTS light work correctly before you did the work on the truck?
I just had a 97 that had some work done to it do the same thing. Started up and ran for about 20 to 30 seconds. Then died and wouldn't start back up.i ended up pulling the batteries to charge over night, pulled the fuel filter changed it drained the bowl poured in some fuel put in a fresh filter. Ended up checking the oil level to find it was a little low topped it off and cranked on it several times and it finally popped of very rough but cleared up after about 15 seconds.
