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I guess you could have a wrong size relay. In my experience a relay is either good or bad and the flash interval is a function of correct wiring. Bad connection = slow flash.

bison wrote:

2. Then when I turn on the turn signal, they flash very slowly, like every 2.5 seconds, which seemed extremely slow to me, almost doesn't qualify as a blink. And they don't blink in unison, when the fronts are on, the backs are off.

That is how the "trailer adapter" relays work that allow an extra set of rear lamps to be used as both brake lights and turn signals. It sounds like it is the wrong kind of relay. You want the stock kind which is a simple metal strip flasher (usually a small cylindrical thing), not one of the black box kind with relays inside.

There are various relays at the autoparts store. The one you want has the pins in an L shaped configuration and only the two pins. You want to buy a HEAVY DUTY relay as I stressed in a very detailed manner because if you do not, the problem you will have is just as you describe. It is used in the manner bountyhunter descibed... to handle extra lights; in other words, a different wattage on the circuit than originally designed. If you choose to not do this, the blinkers will blink erratically and slow. You may still have bad grounds. Check/clean those carefully. That will also cause the slow/erratic blinking.