I bought an old used rally car. Is it grandfathered?

Short answer: maybe.

Long answer: Maybe, with several "it depends", and a couple more maybes and finishing off with a "probably not".

The biggest issue is the cage design and construction. Some older cages which would not be legal for new builds are allowed to continue racing. If the car was legal "back when", and it still has the old log book, it may be legal to race now. However, some of cage design updates were mandatory, so a lot of it depends on when it was built and what's been done to it since.

If the car was never issued a log book, you're out of luck. Let's say you found a rally car that was built in 2001 and legal according to 2001 rules. It sat in a garage right after the roll cage was finished, and it never got a log book. If you want to buy that car and come race with it now, you can't say "It was built in 2001 so you just grandfather it according to 2001 rules, right?". Nope. All new log books meet current rules. The old log book is lost? Sorry, we can't do much with that either.

You should call (not email or IM or facebook or text, call) the closest scrutineer before purchase and discuss the car in detail as well as have them look at pictures of the cage.

The good news is that most cages can be brought up to spec by welding in just a couple tubes. This is why you need to get in touch with a scrutineer.