I'm glad you asked...
The very first car stereo that allowed you to bypass the internal amp and use a separate amp was the Westport in-dash AM/FM Cassette Deck, Model HP-100 and was used in conjunction with the Westport Amp, Model HPA-40 (25W x2). I bought this set up for my Pinto.
In the rear passenger area, there were these large recessed areas that doubled as arm rests. I made some panels that went over these areas (one on the left and one on the right) and installed 8" Radio Shack Woofers, and separate Phillips Midrange speakers & Tweeters, all tied together with Radio Shack 3-way crossovers and stuffed some sound deadening filler in their and even made a port. And all I can say is that for that time, probably around 1975, it cranked up so loud & clear that while blasting it in a friends driveway, his neighbor across the street called the cops (and this was in the middle of the day)...!
Now, I've had bigger and badder stereos since, but always had a fond memory of that set up...