Reverse lights on with headlights, but not in reverse?

Earlier last year I replaced the stock headlight and taillight assemblies with after market equipment as well as installing a backup camera and a new stereo head. I did the work myself and had no issues. That is until the magnum had been left outside a week with heavy rain fall. I found out the hard way my taillights were out. The fuse kept popping. I couldn't locate the short, so I took it to a mechanic that claimed to do electricals.

It came back in worse shape than I sent it. The fuse issue was over. They said they couldn't find the short. At a guess, with the heavy rains that week- moisture got into the mechanical splices that were provided with the parking lights (parking and taillights are on the same fuse. I have gone in and replaced all of those splices with solder joints covered in shrink tubing). The shop said they couldn't find the short, but my reverse lights came on anytime I turned the parking lights or the head lights on. If the lights are not on and I put it into reverse, the reverse lights do not come on.