98 540i with nav that doesn't work and other problems

I recently picked up a 98 540i with navigation non dsp and the nav screen doesn't work. It just looks like an old tv without any input connected to it while it's on. The radio and CD player do however work, kinda. The head unit doesn't shut off but it will stop pushing any sound from the speakers after roughly 3 minutes of sound. But if I turn it off and turn it right back on it starts making sounds again.

So I was going to swap in the old jvc unit I have in my e34 to see if it's the head unit or the amp and it's a no go because the adapters are all sorts of different.

I don't want to retain the headunit or the nav so where can I find an adapter to wire up a new headunit and does anyone have any idea if it's my amp that is bad or just my head unit? thanks for your help.

98 540i with nav that doesn't work and other problems