You could have a bad wire on the power supply side, you could have a flaky power supply or a marginal board. If you are not using the power supply then you already may have a clue. I really doubt you can fix this yourself. It's not easy to troubleshoot something like this based on what you wrote. You need to be able to test a lot of places with equipment
and schematics. Someone that knows is going to have to verify you ever had it hooked up correctly first then work
through the alternatives.
I would haul the whole thing in to a shop and let them go over it. It won't cost that much to have it examined and they may be able to tell you if this unit is even worth repairing. Many of the replacement parts
are sort of spendy. A couple bad parts
or a bad display and you could have had a new one. It may be worth repairing but for a radar
I think you want it checked out so you can fell good about it being reliable.