Now you know why it wasn't mounted in the first place....
You may have created an earth loop whereby the system is now earthed via its own wire and also via the casing you have attached to the mounting points. This may generate a small earth voltage that upstes the system. I may be talking rubbish here but this was a problem on stereo systems I used to build, the voltage resulting in a loud hum in the speakers. Disconnecting one of the earths sorts it out. It may not apply to car units but unless you've pulled a wire it's a last resort and easy to check.
Dismount it again and see if it works. If it does the mounts will need insulating.
TonyB