Rear running lights not working?

We purchased the van listed above used and noticed right away that there was an electrical issue. I had the battery and alternator tested to see if they were bad and was told the battery was. Went to replace the battery, the new place tested it and said it was fine. Husband goes to hook it up, blows half the post off the battery. Takes it in for a new one, turns out it wasn’t the right size. Get a new battery, the issues stopped, but, the radio didn’t work, interior lights aren’t working, the lights behind the gauges aren’t working but the mileage and engine light are. At first the heat wasn’t working. After replacing a fuse, I can control the fan speed but not the temperature or position of heating. The rear windshield wipers won’t work along with Defog. So, we go to the fuse box. Replace a 20 fuse and the radio is working. So, I have radio and heat
A few weeks later my turn signal and brake light are out. Replace those and they are working. In the process I noticed the mount light was out, replaced that. After replacing all that, I have front running lights but no rear running lights. Brake lights and turn signals work fine.
Short in the wire? Is the fuse box shot? There is only one fuse box on these vans, correct?
I’m at my wits end. I have a vehicle with a blown engine, a vehicle with a bad cam shaft, and now this. I bought this vehicle to get us from point A to B! Other than described, the exterior is rough, but it only needs an O2 sensor and strut work.