Just bought a 156, got some lighting problems

[vxmatt]

I've recently bought a 2004 156 2.0 jts and I'm having a couple of problems.

It has factory fitted xenons and the drivers side isn't working, I've tried a new bulb with no success so its obviously the ballast. I've got a h7 HID conversion kit that I used on an Astra I used to own. Is there anything stopping me from bypassing the factory ballast and bulb and using the conversion kit instead?

Also the brake lights keep staying on after the key is removed, searching has shown that its likely to be the rev counter. Do I need to replace it with one from the same year and engine or will any rev counter work?

Thanks in advance for any help.
Matt