True and true. Make sure also to separate the sub chamber from the mids as they can effect the output of the vocals. Also, directional placement is a factor because the sub, being in a location known to lose gain at midbass response you may want to offset them to load towards the underdash area, or even install them there if possible. Front stage bass is amazing once you get it right, but around the 120-250hz range it can cancel very easily if not placed correctly. It might be recommended to still have some low end in the rear for reinforcement.
What I would do is, have the 6.5 subs carry the kicks and drums, and the rear fill carry the chest hits. If in phase at the listening position, it can sound quite natural and blend the lower end to the vocals really well if done right.
As far as procedure, you can measure the dips and peaks of all frequencies at different positions in the vehicle to know what to do with the designs response at the given positions of the source and receiver.....much like in an ht setup for placement and design.
Just figure out the response gain, take a sub that will fit and cover the range of interest, and design it to perform for what it needs to in order to get the best response possible. It might be a good idea to experiment with Aperiodic chambers. I have done this in my setup and my doors response goes down to 38hz quite well with the two 6.5s in each door.
Sometimes people fiberglass pods to make them fit better, and that's an option, but if you are to seal them off, do not expect superb subwoofer performance. Aperiodic is the way to go in your case also it seems.