I haven't heard of any "mods" for the FM transmitter on the SL2, but you can use a wired FM direct adapter.
FM Direct Adapter (Wired Relay)
It's a little adapter that plugs your Sirius radio directly into the FM antenna (instead of broadcasting the signal to the antenna). Only $20 on TSS-Radio.
The wonderful DELCO radio dont have that option .Its FM/AM radio only .They do have a BIG RADIO though,ill give em that.With all that surface mount electronics its STILL BIG!I had the same problem with the reception on the SL2 dock. I ended up getting and installing the direct connection adapter. A little spendy, but well worth it.