There is a shop called John's Mustang it's kinda small but they have some great cars in the garage and they can get all kinds of parts for you. I have taken my car there a few times. They are only a few minutes away from Downtown. Also there are a few local car shows that go on during the week. If you need anything at all while you are in town let me know.
Are there still Mustangs rolling through the u-wrench-it junkyards along the boarder? While I lived in San Diego from 1988 through 1998, it seemed there was at least one new car a week. Here in Kansas City, there's none.
You absolutely have to check out Mustang Auto and classics in El Cajon (SD suburb). Not only do they buy/trade/sell classic mustang parts, but they still sell Mustangs on a lot, like a used car lot. Only they deal exclusively in 65-73 stangs. Man it sure it a weird thing when you look at a lot of 35 year old used cars. Awesome place just to go see the cars they have on the lot.