2000 GMC Yukon Left front signal flashing rapidly

You have two conflicting symptoms that could be related. First, rapid flashing is the indication a bulb is burned out. Check front and rear, but be careful in the rear because there's usually two or three bulbs, so it's easy to overlook one of them. Compare both sides to each other.

Incorrect brightness can be due to a bad ground on the other side, but then that other side will have a dim or inoperative light. The combination of symptoms can also be caused by using the wrong bulb. That shouldn't be a issue with a 2000 model, but if you still have bulbs in back with the round brass bases, check that a socket with two contacts has a bulb with two contacts, and a socket with a single contact in the center has a bulb with a single contact.