The latest source of Transfer Relays on ebay are also easily modified for use at 12v instead of 24v.

*** Information Corrected 13/5/2017

Ace Technology
Model : GW21-A1
P/N : RF402629

This is basically the same as the other relays, as standard the coils are in series, we change them to parallel.

Here's the Relay:

Relay PartNo

Relay Conns

To modify for 12v operation there are 3 steps.
1: Cut the track between white wires on both white connectors on top pcb.
2: Solder wire links under these headers to parallel the coils.
3: Swap three wires (blue,brown and white) wiring in the white plugs.

Before Track Cut

After Track Cut

Wire links added

Modified Wiring

Changes to the plug wiring:
The Right white wire position is not altered.
The plug leading to the coils nearest J1 can be left disconnected if you do not require the 'transfer' facility, this saves power if you are just using it for a 2-pole switch.

Wiring Changes

Example use at mast head