the e36 has three separate antenna and the stock headunit rotates between them to get the best signal. In an after market HU, it defaults to the one in the rear glass. If you have tinted windows, you can forget about clear reception, but you can still pick stuff up.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>