Zach and I are trying to tell you that the two constraints of the argument are mutually exclusive.
Nothing about modern AM auto radio design will give the reception (RF and IF-wise) of an old radio for great AM analog reception.
Unless you find one with switchable bandwidth that can tune in 1 khz steps or continuously.
Sorry but it's true. You probably thought I was trying to be funny, but I am dead serious.
So find the HD compatible unit that suits you, use it for FM, FM HD, AM HD, and when you want to use a real AM radio
turn on your old AM radio, and select the Aux input.
Of course, you'll have get an antenna splitter, and have a good ground to the auxilliary radio.
Even then the noise from an HD head may inject noise back out the antenna jack with as much as is going on such devices.
One of the reasons I keep driving my ancient cars is for the great AM radios.