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If the CRT equipped Riv's used the Control Data Module (CDM) for the tuner or tuner and amplifier (no amp with Bose), it would actually be quite easy. The most common application of the CDM (with amp) was the C/K pickups from '88 to '94. Any of the other GMs with 'weird' head units used them too. Some are labeled as such;
This was from the trunk of a '90 Seville (no Bose). The head unit looked like a standard 1.5 DIN GM, but the tuner and amp were actually in the trunk (many DeVilles and Eldos too).
Here is one with a higher end radio where it is actually attached to the radio;
I had this one apart to install an Aux input.
The Bose units only have an outline of the heat sink shown in the above picture, as the internal amplifiers are deleted and the unit only outputs a low level signal out to the individual Bose amplifiers. Otherwise, it functionally the same as adding an aux input to any other CDM equipped GM vehicle.