Typically the key only locks the radio in the bracket in the engine, this is my experience with Motorola Spectra, Astro Spectra, and MCX100 versions, I believe the AAR requires a locking mechanism be included in the design (just as the 'UHF' connector, four-pin power connector, six-pin hand mic connector etc etc).
If you've taken all the visible screws out, then it should open up. the Aerotron units I used to mess with involved only a couple top-mounted screws to get inside the unit, more from there separated the head from the chassis, and the power supply from the radio portion.
The Spectra/Astro spectra have four hex screws on the bottom, this opens it all up and all main units come apart without further dismantling.
The MCX100 version requires the removal of four security torx, and about 10-15 other torx screws to get the cover off, then it's a mess to get the rest apart (and this one is from the early to mid '80s.)
The point is that all radios are different and that I've never had a problem opening it up any unit as long as I've removed as many visible screws as it looks like there are in order to take off the covers or break it apart into individual components. Now, I've never messed with a trackstar unit in particular, personally, I don't think there is much interest in general as its typically of little use (no scan, not easily programmable to the ham band, etc...) or whatever the case may be.
It should open up doing what you've done, and maybe by now you've found success. If not, then it's probably getting caught up by an internal connector, corrosion, rust, whatever that a little prying should give access, if to nothing more than to isolate where the sticking point is.