Hi.

A relative has a 500 bought used a couple of months ago.

Steering wheel shakes at idle, and when revved it stops. Haven't checked if it changes when a/c is switched off. But didn't remember my mom's did it, when she had one 9-10 years ago.

Is it an engine mount or?

Thanks.

An old thread said it was the AC..

It will be interesting what you find

Start by checking the integrity of all the engine mounts; after that, you're looking for any component that rotates when the engine is running that's gone out of balance, perhaps because of a failing bearing.