Sure you can. You just need to find more dishonest salesmen. Or dumber customers. Or, offer the transmission as an "upgrade package" and contract out the installation.
Or, if you're my old company, you tell your employees that they need to sell X number of cars, period, and then let AR and the Purchasing Departments figure out how to install a third-party, aftermarket transmission.
It's worked for my old company so far....they're still in business. Sooner or later though, some other place is going to come in and absolutely eat their lunch. The execs will then be left with a bankrupt company (or one that's just much smaller, there is such a thing as "brand loyalty", no matter how much the product sucks) ...and they'll wonder what happened.
My stuff is totally weaved thru the other parts of the design. It is not stand alone plug-in stuff. It is common to purchase stand alone IP for chip design like CPU cores or DDR2/3 or USB modules. My stuff connects all that stuff together and controls it.