Hello,
First, you should not be having this problem if the cart is not moving unless something is either causing the OBC to transmit the interference or there is a nother problem?? I would check and make sure that the accelerator switch is not allowing any current through when not depressed. I would suggest re-checking every wire connected to the converter, stereo, lights and any other accessories is not in any way connected to the chassis (through parts mountings, wires, screws, etc.).
If you don't feel comfortable doing the following test, don't. If you do, then check this:
With the stereo turned on, CAREFULLY check with a voltmeter from the positive post of battery#1 (should be where the main red cable that runs the cart is connected) and the cart's chassis.
There should be 0 voltage if everything is isolated. Remove the meter leads and turn-OFF the stereo.
IF you measured a voltage above,
disconnect one of the battery jumper cables between two of the batteries (breaking the battery string series connection).
Remove the + red INPUT wire that connects the converter to the battery string where it connects to the battery),
reconnect the battery jumper cable and check for voltage from the positive post of battery #1 and the chassis (as above).
If you no longer measure a voltage there, that means the converter/stereo is not properly isolated from from the chassis. If the voltage IS still there, then something else in the cart wiring is not properly isolated from the chassis.
Disconnect the battery jumper cable again. Reconnect the converter + INPUT lead to the battery (original location). Then reconnect the battery jumper cable.
Ref you question about the booster, probably not since the interfering RF from the OBC is more than likely being received by the antenna just like the FM broadcast station. You could try it, it might help some in the signal is only coming in over the stereo wiring, but I would not bother.
Ref the antenna, NOT REALLY. You could try re-running the wire antenna in another direction. That may help. For a test only, can you run the antenna outside the console and roof and somehow hold it vertically and see if that has an effect on the issue?
Let us know what you find...
Good luck!