Before I take my G37 into the garage to have the sunroof drain replaced. Has anyone had success injecting a degreaser or some other kind of liquid cleaner through the sunroof drain hole to clear it?

 

What are your symptoms? Is water leaking on the floor or from the headliner?

 

Onto the floor mostly.

 

Most people conflate a "clogged sunroof drain" with the leaking "firewall grommet" issue. They are not the same. If the drain tube was clogged water would potentially overflow from the top (headliner) just like it would in a sink or bath tub. Being your headliner is dry and that water is leaking onto the floor, you either have the firewall grommet issue or the windshield cowl issue. Running any type of "drano" will not help.

That said, there are tons of threads on these issues:
If water is on floor, see here: Sunroof Grommet Leak #1, Sunroof Grommet Leak #2, and tons more.
If your cabin air filter is wet, check here; Windshield Cowl Leak
Being you have a Coupe, if water is dripping from the door, see here: Door Pillar Leak

 

If passenger side leak, I vote windshield cowl.....

 

I don't think you can get a better response than this. First would be to take it upon yourself to diagnose the where and when. Once you have identified the fault, you move into the repairing steps of this fault, if you follow these correctly the problem will be fixed.
Drain cleaner will not work.

 

So what did you do? If it was sunroof drain plug issue, the ones behind the air vents, I've posted on here before how with the right tools it only took me 5 minutes to fix. Read in other post where people went to the dealer and had to pay damn near $1,000 to have the dashboard removed to get to them.

 

I really recommend going ahead and replacing the 2, and doing the modification with a plastic straw and cutting the flow restrictor. Its your best bet to prevent this from happening in the future

 

Screw Nissan's crappy firewall drain setup, bypass the firewall drains and tap into the HVAC drain:

 

Irregardless of any hacks, tricks, or continuous drain grommet "revisions," the bypass is the only guaranteed, surefire method to avoid this issue.

Just as with the plastic heater hose coupling, this should addressed BEFORE it becomes a (costly) problem.

It's the only solution I'll ever recommend. But, that's just my 10¢.