Yellow Hat is more of an accessories store - a japanese chain!
They have a shop in Times Square Mall (just after Mall of Emirates on Sh Zayed Road, if you are coming from Dubai). They have a pretty good demo area for their music systems.
They have their own workshop or hook-up with workshops to do installations. Don't know if they have an outfit in Abu Dhabi.
Everyone in Abu Dhabi normally goes to Musaffa area to get car stuff. There are also a bunch of shops on Salam street as you get closer to Abu Dhabi 11th street upwards to corniche.
Warranty void? Depends on what you do. I don't think a sat-nav system installation should or would void warranty of your engine and other parts not effected. You might want to read the small print though. Or just keep your old music system handy in case you need to temporary re-install it