Clearing the way to merger approval, XM and Sirius agreed to pay a $19 million fine for violating radio transmission rules.

The move is aimed to end a 2006 inquiry by the Federal Communications Commission into radios that broadcast some signals in adjacent FM radio waves. The FCC stepped in when satellite radio transmissions interfered with conventional radio channels.

The two companies agreed to pay the fine – XM will pay $17 million and Sirius will hand over $2 million – and correct their radio and broadcast transmission problems.

The settlement is expected to pave the way for a 3-2 FCC approval. As the review entered its 18th month, it became clear that the agency was split along political party lines. The swing vote on the deal is Commissioner Deborah Tate, a Republican appointee, who is reportedly close to filing her vote in favor of the deal.

The two FCC Democrats, Jonathan Adelstein and Michael Copps, oppose the combination. Copps Monday gave an unconditional no vote on the deal and on Wednesday, Adelstein voted no after he made no progress getting the companies to agree to conditions like a six-year service price freeze and mandatory public access to a quarter of the combined companies’ airwaves.

Sirius and XM filed for a merger review with regulators in February 2007.  Just over a year later, the Justice Department approved the deal, saying that conventional radio and MP3 players like Apple’s (AAPL) iPods were sufficient competition to keep Sirius from setting high prices.

The FCC was a more difficult hurdle, in part because the agency wrote the original satellite radio charter, which specified that the two radio wave licenses should be in separate hands to foster competition.

In an effort to ease the concerns that the satellite duo would use its monopoly status to gouge consumers, the companies proposed new price plans like 50-channel and 100-channel a la carte  offerings that would cost $13 and $15 a month, prices that would stay static for three years. XM and Sirius currently charge $12.95 a month.

The companies also promised to set aside 4%, or 12 channels, for outside access like public service and minority programming.

But as the merger approval process dragged on for nearly a year and a half, the two companies saw growth cool and losses mount. And as new car sales fell, the satellite radio sector suffered as well. Investors worried about the perpetual need for new financing and the prospects of recapitalization pushed the stocks down. XM shares are down 23% from the pre-merger announcement level and Sirius has dropped 37% in the same period.

Looking ahead, if the companies manage to bring their operations together and find new sources of cash, the new entity will likely take aim at lowering high-priced programming deals like the five-year $500 million contract with Howard Stern, which expires in 2010.