You may need a D/A convertor, but probably not. The usual situation is that cars made before September 2002 with DSP require a D/A, others do not, however there are plenty of exceptions to the rule.

If you already have a CD Changer in the car then you can find out for sure by looking at the connections to it. If you have a 3 pin connector and 6 pin connector then you do NOT need a D/A, if you have a 3 pin and single thin coaxial cable then you do need a D/A.