Replacing windshield antenna with fender mounted Antenna

[SJ]

Patient: 90 100q
Symptom: Original Delta radio, poor radio reception.

Since I bought the car in 1999, I have had lousy radio reception with engine
running, especially the AM stations.
The radio reception clears up with engine off.

Instead of screwing around with the amplifier in the back and trying to
figure out what the problem is, I'd rather install a standard rear fender
mounted antenna.
Are there any issues with the antenna cable connections at the radio?
Do I need an antenna cable interface adapter?
Can I just disconnect the front windshield antenna from the radio and only
use the rear antenna?
Do I need a termination resistor or short at the disconnected front antenna
input jack?

Any BTDT would be appreciated.

SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q