Help!

Im new here, but am looking for any kind of guidence when it comes to electrical. Since my fiance and I installed his system in his car, there has been loads of electrical problems. Right after installing and finally getting it all to work properly, the electric windows stopped working (although the sunroof still does). The next day, the tac stopped working as well as the gas gauge (the speedometer didn't work when we bought it, but still intend to fix it). I don't know anything about electrical, but he checked everything that he worked on, and near, aswell as the fuses. Nothing seems to be the problem. Is there something in particular we should be looking for? Is it common with the model (1982 733i)? If anyone has some insite, please reply!!!


Specs: 1982 BMW 733i, Automatic, 3.2L, RWD


Thank you for your time!