I would say check all your connections first. 99% of all electrical problems are a bad connections. You should be able to get to the wires under the bed cant remember sorry my trucks are flatbeds much easier to get to fuel tank.
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98 3500 excab 12V 5spd 6ft flatbed under construction
95 3500 12V 5spd
97 3500 12V 5spd cab chassis 4x4