I need help with a possible electrical issue with my 2000 Jeep Grand Cherokee Laredo.?

I bought my Jeep in 2013 at a great price. I had it inspected and the guy actually told me that if I didn't end up buying it to let him know 'cause he would! He also made my day when he said "You know it's got a new motor, right?" The dealer I bought it from wasn't actually a used car dealer but sold hauling equipment such as flat beds and horse trailers and had taken the Jeep as a trade so I ended up with a great deal. I've only ever had one issue with it from day one. It doesn't affect it as far as driving or handling but it's just annoying as can be. I always buy the Haynes manual for any car as soon as I get it but I can't find the issue in it. My husband is a pretty good shatetree mechanic and I have no doubt he could fix the problem...if we knew what it was so I am really hoping someone out there has some ideas.

Here goes: To play the stereo or run the air conditioner or get the interior lights to work...any of the interior lights including the dash assembly or gear indicators...basically anything electronic/electric...the headlights have to be turned on! It's the craziest thing...I have to at least have the parking lights on. As I said, it doesn't affect the driving but I feel silly running around with my lights on all the time. I'm wondering if it could have something to do with when the new motor was installed. Does anyone have any ideas?