yes it's inside positive. the directions on the aftermarket cable weren't that clear (you can attach the additional connector pin to be + or -) so I just made sure to test with a multimeter to make sure the polarity matched the oem power supply.
up here the 24" is $30 more, but it makes quite a difference in the size when you compare them in the store. I just hope the 24" will fit, mine's going in a truck camper and the 24" may be too big :(
Insignia also makes a 24" in a 720p, but the one they sell up here is the 1080p version.