How many can i hook up

i want to know how much things can i connect to the 12volt power cable that is for the car stereo. is it safe for me to connect a screen, and an fm modulator to it.

All depends on your current draw. Check the wire's AWG value (most I think are around 12-14 Gauge). Then find the current capability of the components you are using, and see if it is under that wire's value. You should be fine with the Monitor and FM Modulator. If the screen is an LCD, most only draw 100 mA, and the FM modulator should be under 500 mA, though I am not quite sure. Check the packaging. You can also use a relay if you exceed the current draw. Good Luck.

Thanks for the response. what about if i connect it to the 12 volt wire that goes to the cig lighter. am not a smoker so i wont miss it.

Well, if you check your vehicle handbook, you will see the fuse section. Check which fuse goes to that circuit. If you don't exceed your current draw on that line, you should be fine. Also, some vehicles the cig lighter is on all the time, where other vehicles only come on when the accessory is on. Good Luck!

my cig lighter only comes on when the accessory is on so i am thinking of taking the cig lighter out and looking behind it to find which wire is the 12v and ground disconnecting them from the cig lighter and connecting the monitor 12v and ground to it. Still a newbie so i hope this is possible. But i sounds staight forward.