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But whatever you do, don't hook a lead for TomTom straight into 12 volts. The cigar lighter adapter head contains a 5 volt regulator and wiring the thing straight into 12 volts will literally smoke your TT. I've heard of this happening to a few due to older cigar lighter adapters having a failure mode that puts 12v straight across the output.
I imagine both units use Lithium-Ion or Lithium-Polymer batteries. These are really best only used when no alternative power supply is available, as discharging and recharging these batteries eats away at their overall lifespan. Lithium batteries should be discharged as little as possible and then fully recharged as soon as possible. So while swapping lighter plugs is working in the short term, it'll be slowly and pointlessly degrading the batteries in both units.
Don't be tempted to 'deep cycle' Lithium batteries as you used to have to with Nickel Cadmium and (to a lesser extent) Nickel Metal-Hydride batteries. It will degrade the output rather than improve it.
The cigar lighter splitter is probably your easiest bet. Maplin, Halfords etc. do them, but you'll probably find one cheaper at your local market's motoring stall.
Currently in Horatio's garage:
2010 Laguna III Initiale.