If you consider the word as a countable noun, then there must be different services in mind. Muffler replacement, brake job, wheel alignments. You took your car in for a (one of those) service. The garage performs these after-sales services.
Most of us don't limit the word in this way. To say the dealer provides after-sales services makes me wonder what services they do not provide. If they provide only the three listed above, then where do I go to get my oil changed?
That's why you'll find most often that it's after-sales service (uncountable, a general term, covers everything required). And we ususally take our car in for service (not a service) because we know the repair shop will always check everything else, not just the original complaint, and try to sell us other services. Even if it's just to replace the windshield wipers. It's just another service (countable) that's part of their service (uncountable).