RICHARD: And drive behind me. I don’t want that thing blowing up right in front of the Jag.
DEAN: No problem. Try to keep your electrical system working long enough to get there.
Jag is short for Jaguar, a luxury British vehicle company headquartered in Coventry, originally founded in 1922 as the Swallow Sidecar Company. The current name was chosen in 1945. It holds royal warrants from Queen Elizabeth II and Prince Charles, and has manufactured cars for the British Prime Minister.
There isn’t a good shot of Richard’s Jaguar, it’s behind Rory’s car. I have been informed by a trustworthy source that the car appears to be a Jaguar XJ6 sedan, possibly a 1974 model, based on its hood ornament, chrome bumper bar and paint colour. That’s a classic car known for its performance and superb handling.
Dean mocks it for being unreliable, probably because the 1970s XJ6 sold to the US market is notoriously difficult and expensive to restore and maintain, due to its complexity and numerous issues with the electrics. However, Richard is later shown to be very interested in old cars and has a lot of money (and a chauffeur … hey, whatever happened to Lance, anyway?), so he would be well positioned to take care of an old Jag properly.
You can see Richard is being slightly hypocritical about Rory’s car, since his own vehicle is old, and probably has more safety and reliability issues than the one that Dean has rebuilt.