I have now found a solution for this problem. I didn't want to replace my stock head unit for two reasons.
One reason is that I wanted to keep the steering wheel controls functioning, although I know that _might_ be possible to do if you buy an aftermarket steering wheel controls module.
The main reason though is that this car is very rare here in Sweden where I live, and I want to keep it looking stock. Very rare is actually an understatement, since there are only _TWO_ W-body Regals in this country, one LS and my GS.
To be able to listen to all radio frequencies, I installed a module called a US Radio 0.1MHz frequency shifter/FM Band Expander from Merlin Productions (http://www.merlinproductions.co.uk). They claim on their site that they are the only company making this kind of frequency shifter, and I actually believe them because I tried to find a cheaper one elsewhere and I didn't.
It installs in series with the antenna cable, and then there is a power wire which I hooked up to the power from the cigarette lighter with a on/off switch in the circuit. If i flip the switch ON, and tune the radio to, for example 104.3, the frequency "shifts" 0.1Mhz and when the display says 104.3, I actually listen to 104.4. It works as it should, and now I can finally listen to more than one station!