Car Stereos: 02 dodge ram 1500, dodge ram 1500, stock stereo

Question
do i need to get an after market head unit to install a sub or can i hook is up to the stock one

Answer
You can use the stock stereo.  As a matter of fact, most factory stereos are better than what you can buy in the aftermarket.  Adding a sub/sub amp to a system is very easy, and requires no other upgrades to install.  The sound difference will be definitely noticeable. Hope this helps, Scott