Relay for rock lights

i have rock lights that i wanna set up to a relay the relay only has a positive but it needs both positive and negative anyone have any solutions or advice

 

Not sure what you are asking, a relay typically has 4 or 5 pins. 2 pins are control pins (typically pins 85 and 86 on a standard relay) one needs power and the other needs a ground to turn the relay on. The other 2 are the load you are powering (typically pin 87) and 12v fused power from the battery (typically pin 30).
On a 5 pin relay 87a is the normally closed contacts.

 

Perfect explanation above. OP, if you still have questions, please clarify.

 

A relay is just a switch that can be activated by applying power a coil in the relay. In the pic above, pin 86 and pin 85 are connected to the coil. Supplying power to the coil will cause the switch to close, connecting pin 30 and 87.

As shown in the pic above, you would use a relay to control the supply of power to the accessory you want to control. So pin 87 would supply 'positive' power to your accessory. For your 'negative', you just connect your accesory directly to ground / negative. There is no need to switch the ground. So the accesory positive gets connected to the relay (pin 87) and the accessory negative / ground gets connected directly to negative / ground (no relay needed for negative / ground)

 

When using a relay that has 87a and I don't need it; I put the 12v on terminal 87 and the load on 30. This way 87a isn't live, open to a possible short, when everything is off.