radio antennae are in trunk lid
I think it is all at the back of the car.
The radio receiver in an early DB9 is a separate module located in the truck, communicating to the head unit via the MOST network. The actual antennae (yes two) are located under the inside of the trunk lid under the carpeted cover. To install an in-line FM modulator you will need to insert that modulator in line with an actual antenna. Pop off the carpeted cover and will see there are the two antennae with euro style miniature BNC type connectors. You will need a convertor to a standard Motorola antenna plug common in the US. There looks to be a power source to the antennae too, so maybe you can splice off one of those if needed.
Since there are two antennae I presume the radio receiver module tries to optimize reception by selecting the best signal. So... perhaps if you connect the in-line FM modulator to just one of these antenna the radio receiver should see that strong signal for your AUX/SAT radio input, and see the other for regular radio.
Now of course you have to get the AUX cable to your in-line FM modulator thread into the main cabin...