Did I just waste my money because an IDE cable can't handle the 5V required by the display (3.3V required by the backlight) ? I measured resistance between the 2 ends of some wires, multimeter show ~0.8 Ohms for each.
If the cable really is 0.8 ohms its a dud.
More likely you are measureing losses and errors in your multimeter and its leads - to measure resistances less than about 5 ohms
I recommend passing a known current through the thing and measuring the voltage across it, R = V/I
Loose connections could cause enough voltage drop to dim the backlight, 15cm of wire shouldn't.