KJ kyle wrote:
I don't know much about relays, but I
think this is + and -. so I obviously have power
to the relay, but I wasn't showing any at the fan. I'm not sure where the circuit goes between the two, and it's raining at the moment. I'll update with any findings.

Ok, the 13 volt shows the power to one side of the relay. That's very good. Now it's just a matter of taking a piece of wire tripping the other leg until the fan turns on.
You already found the 12v source. Now, take one piece of the wire and insert into the source voltage (Red Lead of the Multimeter).
Using the relay as a reference. The legs are labeled on the bottom.
85 and 86 are your 12V source
87A and 30 goes to the fan
87 is the trigger circuit. (energizes the relay and closes the switch allowing current and voltage to pass from the source voltage to the fan's
Take 2 pieces of wire and connect 85 to 87A and 86 to 30. The Fan should turn on. it may run backwards, but it should turn on. If it runs backwards, just reverse the connections.
Think of the relay as a 3 way light switch ( 2 Switches control a single light bulb, like the one you would see in a stairwell) With a 3 way switch, their is always voltage running between the 2. However it shares a common ground for the light to be lit. The relay is the same concept, where there is always voltage applied, it's just waiting for the ground to be closed (ECU - Trigger signal) so it can pas the voltage to the Fan (Lightbulb).