Just to let people know in case it's useful. I had the trim motor on one of my mid-90's 50hp Mariner 2-stroke fail. One advantage of twins is that you can compare them; the working one had continuity between the 2 wires going to the motor, the broken one hadn't, so it was obvously a motor problem....
Lesson 1:know how to raise your engine manually or you're stuck with it down which is a problem both for going home and then for getting at the trim motor to repair it - in my case, it involves backing off a screw accessible through a hole at the bottom of the right hand side of the engine bracket, by 5 or 6 turns (manual said 3 or 4), which lets the hydraulic fluid bypass the pump, so it can flow freely and you can lift/lower the motor by hand
Then, on taking the top plate off the trim motor (remove 4 screws and gently lever it off), it was immediately obvious that the wire to one of the brushes had failed. Mariner don't make replacement parts for these motors anymore (at least, not the top plate/brushes) so I was looking at £260 or something for an entire aftermarket motor.
Then I Googled "carbon brushes" and came across Solent Tools, website
www.carbonbrush.co.uk (to whom I have no connection, just a customer - and there are other firms around who do this). Turned out that for about £15 per pair, they take an old brush from you as a sample, analyse it to determine the exact composition of the carbon, and make you custom replacements. Took a couple of weeks, but I now have my motor running again perfectly. Pleased as punch of course...
In case it's helpful, a quick description of the process. Getting the new brushes in was the fiddly bit - the motor comes apart easily by undoing 4 screws on top as mentioned above, but the wires from the brushes are crimped to the supply wires and the crimps have to tuck into little recesses in the plastic bits through which the brushes slide. I used the ends off a bullet connector as crimps and basically squeezed them different ways until they would fit. Even then, its a tight fit in the motor case and I had to resort to a couple of bits of tape (well wedged in place by the case when the motor is reassembled) to make sure the wires couldn't short against the case
I originally tried getting the top back on - which requires getting the brushes either side of the commutator - with the motor in place as I wasn't sure what would happen if I lifted the whole motor out, but it was impossible. So in the end I bit the bullet and lfted it - big relief, all came cleanly except for a couple of washers that were easy to get out and slip back over the motor shaft. Then with the whole caboodle on the work bench, it was a lot easier to use a long thin thing to push the brushes back outside the commutator while easing the top plate down
So (apologies if you know this, but this is for any who don't, as I didn't before now) if your tilt motor quits, before lashing out for a whole new one, you may want to have a go at fixing it - brushes may not be available from Mercury, but they can be found. Saves a lot of money, never mind the pleasure of the Bob the Builder ("can we fix it?") moment!
Anyway, hope others will find this useful.....
but same comments regarding reassembly. 
Sula
