I haven't seen this posted anywhere else, but it might help your situation.
I have the Sirius PNP2 with the car mount and the FM tuner. (got in on the Howared Stern deal, even though I have little intention of listening to Howard Stern!)
I've been experimenting with the FM transmitter, and I've found out the following:
If you connect a plug in the jack labeled "FM Out" and run the center conductor to an appropriate length of wire (I estimate 850cm), you can greatly enhance the range of the FM Transmitter.
If you were to use Coax wire between the SIR PNP2 and this homemade "antenna" you can move this signal to another location (like moving the output of the FM transmitter to the back of the car, rather than the front).
Or, with sufficient height, you could say increase the range enough that all the radios in your house could pick up the signal...
Some other things I observed:
From what I've read, the FM Out has a DC voltage signal present. This doesn't present a problem in an open antenna configuration, but if the two leads get shorted to one another, as in a Coil, or other antenna configuration (like a loop), the FM transmitter will stop transmitting while the short condition exists. I would insulate the antenna to prevent shorting. Doesn't seem to hurt the unit, but didn't push my luck either...
I'm using a 1/4" jack to F-Connector adapter. I may try using a DC blocking connector, and see if that helps with the shorting problem. (The antenna I want to use is a loop).
If you plug a connector into the AUX out, the FM OUT apparently stops working (so you can't use BOTH FM Out and Aux out at the same time Darnit!).
Co-workers who can only get one station on their radios love having a second choice on 88.3 Mhz, some even go out and buy their own satellite radios!!