I am begining to get some headliner separation and was windering if anyone has tried successfully to inject some glue behind the liner without removing the total unit from the car to complete a fix?

Thanks

 

Headliner separation usually means the backing foam is deteriorating. If you try to glue it, even if you are successful, you will be fighting a constantly losing battle.

It is best to bite the bullet and have it recovered. The good news is upholstery shops can generally do this for a fairly modest cost, or if you do it yourself you are only looking at around $50-$75 in materials.