why no solder?

because with solder you are adding another metal. dissimiliar metals and water cause electrolosis and corrosion,

pre-tinned wired is solder coated no?

that is not the reason, soldering creates a solid connection that will break from the vibration in a boat, that is why you have to use stranded wire not solid so it can flex.

each individual strand is coated they are not all joined together

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The problem with solder is that if you use the wrong stuff it has acid in it. Eventually that will cause corrosion. If you use pure silver solder then you will never have an issue. The stuff is also very good because it has a lower melting point. I get it from Radio Shack. It is very thin stuff. Once you use it you will never go back to 50/50

I was thinking the same thing I have never had a solder heat shirk connection fail, I have had crimp heat shrink fail though. Just me experience I guess.

The stock wiring for a mercury trim or tab pump is 3 conductors, the power is supplied from the pump end. You don't have to do it that way, just wanted you to know what the stock setup is. Also, I would run at least #10 to main panel. You never know what you might want to add later. Make sure your main feed is fused or has a breaker at the source. Same for the stereo. A short in a heavy gauge wire can get real ugly.

Does the trim limit wire into that? or do I need to rig that circuit myself??