bulb earth please which is it

i have an earth problem to the nearside headlight causing it to be dim. a i have cleaned up and replaced what wiring earths i can find on the body to no avail. please can somebody tell me which of the 3 terminals on the bulb connector is the earth - there are 2 wires that are red with bits of blue on and one that is black with bits of red on? and the earth is??????????????
 
http://www.ebay.co.uk/itm/CAR-VAN-A...228?pt=LH_DefaultDomain_3&hash=item3f434a115c £2.70 posted, might save you a load of grief

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