Electric mainsail winch upgrade, wiring?

My 370 has two electric winches, but they are the two aft.

I would recommed that you install the electric winchs there instead.

All my halyards are long enogh that can use the aft winches also.

(But I normally hoist the mainsail by hand, standing next to the mast, this is much faster).

With aft electric winches, you get the benefit for mainsheet and selftacker sheet also. I always sail with the sheets next to the helm and can adjust sails not leaving the wheel.

When my wife hoist me up the mast, we use the main halyard and it is just with a push of the button.

Wiring will be much easier also.

Just to add to the sum of knowledge, although I've a powered winch in the cockpit I much prefer to use an EWincher2 (EWINCHER | Electric Winch & Winch Handle for Sailing Boats).  Unlike a powered winch it provides clear warning of a snag that could become a breakage and runs a full days sail on one charge including numerous main hoists/reefs.  While still less than many electric winches, at about £2000 its not cheap but it makes every winch 'powered' and there's a new less-powerful but less-costly version out now too.  In practice its size has never been an issue and it saves a huge amount of amps being taken from the bank as charging can be saved until when shore, alternator or solar power is available.  

Might not suit everyone's type of sailing but it works well for us as 'liveaboards' for 5-6 months in northern European waters.

Where have you fitted the relay and activator button. I am considering putting it at the helm position but it might be awkward in some situations, and handy in other.