I had an older Sirius PNP Tuner that I just used the built in FM Modulator and it worked fine for me.
The old unit died (froze-up on the welcome screen) and I bought a new one about 8 months ago, the new ones have no signal strength from the FM Modulator thanks to the FCC, could only get static on the radio.
I didn't want to tear the dash apart so I bought a FM Modulator from C. Crane Co. , It works great and comes with a transformer to use on 110 AC in the house & takes 2 AA baterries for portability.
I also got their 12 Volt Car Kit and plug into the 12 Volt Cigarette Lighter Port which is key on hot.
Reception with the Crane is far superior to the old units FM Modulator, I have had it on the same Frequency since I got it and have not had any bleed over problems
Unless you are into very serious sound quality, Check out their web site lots of electronic gadgets