6.2L diesel glow plug wire

:-) unfortunately not a very helpful description. There are close to a dozen wires that fit that description that are part of the glow plug system but I will assume the ones are part of the control relay. It does sound like the wire is broken internally. When you jiggle it, it makes contact sending power to close the relay. It should get worse when cold because most metals contract when cold, opening the gap in the broken wire. If the wire is short (in length) just replace it with a new wire of the same diameter. If it is the control wires (I beleave black and pink in color) they send power to close the relay but be be careful to use the same plastic connector with the new wire there is a resister between the two wires that gives a little power flow at all times. This is required by the glow plug controller (magnesium tube looking thing sticking out of the top back of the drivers side head. If it is one of the large diameter wires (supply power to the relay and power to the glow plugs) then also replace these just be careful one of these is hot all the time. You may also want to pull the black connector off (depress tab and pull up) and make sure the contacts are clean.

good luck, mark