Electrical problems

I have searched this forum many times for answers and have not been disappointed. I have a '99 Blazer with several electrical problems, which are not my strong suit. First the gas gauge started to go crazy. After searching the forum it looked like either a bad ground, or bad sender unit. I have been using the odometer until I felt like messing with it. The other day I took the key out of the ignition and got out of the vehicle. When I opened the door, it dinged at me indicating that the lights were on, or the keys were still in the ignition. Neither was the case, so I just closed the door and it hasn't done it since. I took it in for an inspection today and the center high-mount brake light wasn't working so it failed. I took the light assembly off the vehicle and tested it with a 9-volt battery and it worked fine. Both of the other brake lights work fine. Any ideas? Are they related at all?

The very first thing I would do is Check all the fuse's then make sure you have a Fully Charged Battery with CLEAN cables, Then check all your ground straps on the truck. Process of Elimination. Don't rule out a possible bad Ignition wiring harness either.

Thanks, I'll crawl under and take a look.