I put a non bmw radio in a 1990 535i radio and it still doesn't work, any ideas what to do next?

by replacing the battery the anti-theft code requirement has been activated
check the radio forums and comments on this site as there are ways to recover the code

If your losing coolant water pump may be leaking, not enough coolant will make it overheat. If not that, thermostat may be bad stuck open or shut. Hope this helps

Sounds like you have a dead battery. Try recharging the battery or get someone with jumper cables to give you a jump. If your battery dies again after driving for at least an hour you likely need a new battery. If the battery in the car is the original, it's time to replace it anyway.

try to get a fuel pressure test or get it put on the machine to see if the
crank sensor or cam sensor is going bad.

The problem sounds like the battery has run down. Even though the ignition is off, things like clocks and the radio draw small amounts of power to retain their settings, and the instrument cluster doesn't draw as much power as the starter. It usually takes about a month or so to drain the battery. Try jump starting the car, or put it on a battery charger for a while.

the fuse for the radio is under the hood next to the battery

CHECK RADIO FUSE

just because a fuse "looks" good doesn't mean it is, locate the radio fuse and check it with a meter, if you don't have a meter, replace it anyway ,, what the heck, a dollar fuse vs what the dealer wants isn't such a bad idea

It is not in anyway possible.