relay to boost signal?

How do you know when you need to use a relay to boost a signal? For example I want to use the Dome light output which is a - 200 mA output and my system uses a ground to turn it on. Can I just hook this wire to the circuit or do I need a relay to boost the ground?

It's borderline, most alarms aux outputs are on their limit with that current requirement. I would use a relay, output (if neg) to 85, fused at 3amps, 12volt + constant to 86, good ground to 30 and 87 to your dome light. If pos aux output (very rare), send to 86, good ground (which technically should be fused at 3amps) to 85 and 30, output from 87.

When you say fused at 3 amps would you install that on pin 86 or where? Also how do you know when it's a good idea to run a relay and when you really don't need it?

Also thank you for the reply any information you can give me would be great I am trying to expand my knowledge on this subect.

Also I would like to know about diodes too. When do you know if you need to use them or not and how do you determine the proper one to use?  Sorry for the multiple posts but I can't edit my posts yet

The fuse feeds the relay itself and protects it, only the feed to 86 is fused, or on the alternative, 85 to ground..YES fuse the ground side. As an extras precaution, diode from 85 to 86, 1N4004 or similar (Radio Shack, etc.) band side towards 86. When relays shut down, you get an onrush current which can feed back along the aux to damage whatever unit the aux runs from. As I said before it's a combination of knowing the current your unit supplies and the required draw of the end unit, in this case your dome light. Look at the relay section on this site, it's very informative.