This is difficult to answer, since you are actually combining two different elements of grammar.
In both UK and US English, a nonrestrictive clause uses which after a comma:
It's a device you put in your car, which allows you to find your destination.
In both UK and US English, that can be used in a restrictive clause:
It's a device you put in your car that allows you to find your destination.
However, in UK English which can also be used (in US English which can be used, but it's not nearly as stylistically common):
It's a device you put in your car which allows you to find your destination.
More generally speaking, is the clause after car restrictive or nonrestrictive? In other words, is the fact that it allows you to choose your destination essential information you have to convey—or is it merely additional information? Can the main point of the sentence stand on its own without anything after car?
If it's essential, it's restrictive (and there should be no comma); if it's additional, it's nonrestrictive and there should be a comma. And whether you use than or which will depend on what it is and where you live.