Stuck on ‘Grass Roots’ mission as Franklin

If you are unable to destroy the car before the mission, and can't move it, you could try killing yourself when you destroy the garbage truck - if you fail a story mission often enough, you get the option to skip it.

I believe you're speaking of the heist called "Blitz Play". This heist takes place in the southeastern part of Los Santos, if I recall correctly.

The getaway car has to be parked somewhere arbitrarily far from anyone's house, and far from the heist location as well. You'll get a little pop-up message that will indicate you are too close to one of these locations if you attempt to park the getaway car there.

There are some other tips about the getaway car's location that I'd like to give, but I don't want to spoil you if you'd rather not know. There's nothing major here, but I'll give you the option not to read them at any rate :)

Close to the end of the mission:

You'll have to evade the cops in a garbage truck, and then after that head to the location where you parked the car. Thus, the location doesn't matter too much - you won't have a waypoint for it until after the cops are gone, and there's no time limit to reach it.

Once you get to the getaway car's location:

You'll have to blow up the garbage truck. Thus, it's best to park the getaway car in a long enough alleyway that you can park the truck in the alley as well, and then detonate it without destroying your getaway car. The last thing you want to do is get some additional wanted stars at the very end of this mission.