Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
This is nearly always a bad compressor clutch relay. These fail internally and cause a back feed that will allow the blower motor to operate without the key and randomly turn on the headlights... in addition to the clutch on the AC compressor remaining engaged and killing the battery in short order.Go to the under hood small relay box.... the narrow one next to the much larger fuse panel by the battery.... take the lid off and identify the compressor clutch relay. It will be marked on the cover with a snow flake and should be the rear-most relay (nearest the windshield). Pull that relay out and compare behavior... if your problems subside, you just need to replace that relay and you'll be good to go.
I'm going to be stepping out on a break shortly, but if you need more help I will be back in just a little bit for a few hours this evening.