Car Speakers and Thiele-Small Parameter Question

[Johnny2Bad]

I'm in the middle of a build of a DIY sound system for a PT Cruiser. The goals of this build is to maintain a stock appearance with enhanced Sound Quality (SQ) and function.

By "Stock Appearance" in this case I don't mean something that is indistinguishable from factory, but something that fits in with the stock "look and feel', in other words something that the factory "might have done" while retaining an OEM quality to the overall appearance.

So, for example, if a grille must be used because I'm going to cut the factory plastic baffle off which is part of the interior panel, it will be covered with metal mesh with an appropriate curved profile and covered with factory-color cloth. If the car was sold and the stereo retained by me, the new owner could just pop different speakers in there. That kind of thing.

My question is about modelling the crossover for the rears, in particular. I am not quite sure how to treat the "enclosure" in this particular application. I expect that I will be able to properly seal the baffle in my install.

Should I treat the space behind the baffle/interior panel as an infinite baffle, or is this too big an allowance for the (probably leaky) space between the panel and body for a proper crossover model?

What do you guys think, and most helpful would be someone who has some practical experience measuring drivers in free air and then installed this way.

Driver is a Peerless manufactured M&K buyout OEM 5.25" woofer; because it's OEM there is no literature from Peerless available. I have measured the driver in free air with a Dayton WT3, so I have that data available for Theile/Small (T/S) parameters.

[Johnny2Bad]

Here is an image of a typical (not mine) rear pillar PT Cruiser installation location.