Comment by vitaflo
The modem is usually in the sharkfin with the XM radio chipset and GPS. If you can unplug it at the sharkfin that's usually the best course of action. Some cars may bark at you, but mine just says it can't detect GPS if I attempt to use it (which I never use anyway).
Wouldn't it be better to connect resistive pigtails to the antenna connectors on the board? A little more work to get to, but less risk to damaging paint and weather seals, and would do a better job preventing signal leakage. I'm no expert on such things, but will definitely be looking at something like that for the next car I buy.