Cigarette lighter voltage too high (?) - how can I fix it?

[Mercmartin]

Whenever I plug in a mobile phone charger into the cigarette lighter socket of my K-reg W124 320 TE, it (the charger) blows immediately. I've destroyed three so far... I have measured the voltage to 13.4 V, whereas I understand it should be 12 V. This was measured with a number of lights/fans going at the same time. I have had no other problems with the electrics whatsoever and none of the fuses in the box under the bonnet is blown.

I'd be very grateful for any thoughts on what might be wrong and advice on how I can fix this - I need to be able to charge my mobile on longer trips as it runs the sat nav!

Thanks.