Payton, if there is a fuse there why does the pump operate when you hold the trailer switch? The pump still operates in both directions with the trailer switch, it just won't operate from bottom to the trim limit with the throttle button or dash button without having to push the trailer button.
I'm just trying to understand.

Trailer position bypasses the trim limit switch. If trailer position will raise the drive, then there is something wrong in the limit switch circuit. You disconnected the limit switch leads and looped them together but did you check continutity for each of the leads first? From my old manual the up circuit feeds the limit switch which then feeds the up solenoid. Check the up solenoid wire (the small signal wires on the side of the solenoid) on the trim pump to see if it gets power when you hit the up switch if it does not then you must find where you are loosing the 12 volts to the solenoid.


The trailer switch feeds the up solenoid from a separate 12v supply.

trim on handle
 

Not sure but I believe there may be diode's somewhere in the switch circuit where the two circuits are combined as one. Some work that way. If a diode failed, it would affect only one circuit.

I thought of that also, but since he has the problem with his dash switches as well as the throttle trim button, its probably not the case.

there is a diode in the wire just below the base of throttles ,it isolates feed from 1switch to the 2 drives,cost only few bucks from livorsie ,same problem onmy 29 hope this helps

The diode has 12v on both leads out of the switch when cycled. It has to be the in the wiring for the limit switches, but not the limit switch wiring. Know what I am saying? I can take the limit switch out of the picture & loop the pump side of the wires together on both engines & the f'ed upside still doesn't work but the other side works. That tells me it is from the pump to the connections at the limit switch wring harness, right?

Are you sure you are working with the limit switch and not the trim gauge sender? I think the limit switch is on the left side of the gimble.

In my best opinion not testing this problem in person, sounds like you need a new trim limit switch or as most shops call them (hockey pucks or pucks). Mercrusier only sells them as a pair (trim limit switch & power trim sender) for around 90 bucks give or take. This is not uncommon for Mercruiser on a 10 year old boat. The trim limit switch is on the left (port side) and is even labeled. Also check the 2 wires going into the puck and follow them as far to the rubber stops going thru the transom assembly, you should not have any bare wires showing if you do there is your problem and a change is needed.