Yea, by amps i mean the ac inverters for the drive and lift, they should have a little led on them that will blink when powered up, i know the curtis ones do, but the 7400 use another style amp, but i think they still have the led on them, and i don't have a truck in front of me to look at and see.
Is your truck electric steer? I know its not coding out for the steer amp, but unplug the steer amp (behind the lift pump motor) and key the truck up and see what codes you get, i know its not showing a code for it, but possible that its shorted causing the other amps not to power up. (also see if when the steer amp is unpluged do the lift and trac amp led's light up) I would send you a manual but i only have a paper book, B+ is terminal number one on the lift and trac amps small wire connector, check to make sure you are getting 36v at that terminal when k3 relay is closed during startup. It really sounds like a power issue to the amps, or both amps are fried due to moisture, but the traction and lift amps are sealed fairly well and don't seem to have major moisture issues on the 7400's, unless the whole system or bad relay card sent a nasty power spike through the truck. If you have battery voltage at the lift and track amps at terminal 1, and the led's are not lighting up its pretty safe to say they are fried.