I think you may have two problems in the works there. The key-on source should supply both of the relay control circuits (along with PCM, purge solenoid and EGR solenoid). The second two power sources are direct battery
feeds (B+ for short) from the underhood fuse block at Fuse D. These two will remain hot at all times, where the first two will be switched.
But they come in 2s when working right. I figure you have a basic ignition feed problem to the control sides of the relays (and the three other sources above). Recheck your fuses inside the truck to be sure this source isn't being interrupted, which might be consistent with a radio operation gone bad.
My guess is that the single voltage source you're finding right now is B+. That's because of a problem that happens fairly often on the 92-95 Dakotas, where the A14 splice in the left fenderwell harness area rots out. Of the four wires that go into the splice, you can lose each leg individually or all at once.
Here's the general location of the A14 splice...
You'll recognize it as four wires (red/ white) that come together in a hockey stick tape-wrapped splice... not the best way to seal water out. I pretty much expect it to crumble once you get to it!
So... check the ignition feed (inside) and then hit the splice. Let me know what you find!
Talk in a bit,