how do you change the cabinet filter on a 1990 535i bmw and also do any one know where i can find an onboard computer for the same vehicle the amperage draw is pulling my battery down

A '90 5series is old enough to nearly be considered a classic- to keep it running right I think you will have to find a good mechanic who knows these cars and can diagnose and repair it correctly the first time- if you want to do the work yourself, you might wind up spending more time and money than you need to- personally,I get real nervous buying used computers- why I have an analog car-