I assume these are built units rather then individual LED as individual LEDs need ballast resistor but most built units already have them.
Ballast resistor reduce voltage to about 2.0-3.5 depending on LED
Red 2.0 green 3.0 blue ultra violet 3.5
if you need any help calculate ballast resistor
http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng#if you have built unit you need to wire in parallel (view parallel circuit on above link) wire across live and negative as you would a normal 12v bulb but unlike 12v bulb you are unlike to need a fuse upgrade because LEDs take little current. Yes you should be able to link more than 1 unit across the same connection, it take hell of lot of LED even to the same current as a 3watt bulb
You just have to make sure you wire live to live side and negative to negative LEDs (Light Emitting Diode) only work one way but it shouldn't harm them if wire wrong first as they would act like an open switch
Anybody interested
Supplier of LED replacement bulbs I use (sidelights indicators, brake and tail)
http://www.ultraleds.co.uk/