as other people have stated you will not have good luck trying to cut in  between the wires of ceiling heat.  here in oregon there is lot of old homes with ceiling heat.  what I have found works best is to elimenate the ceiling heat altogether but  reuse the ckt and tstat to feed a new heater.   if Access is an issue  in the room the easiest and simplest soulution is to install a cadet C series heater or for a little more money a pick a watt heater.  the pick a watts are a quieter than the c series but either works just fine.  Install it in  the same stud bay as the tstat,as long as there is a full stud  bay there this one of the easiest remodel projects going when you drop out of the stat with your feed you will be fishing it into a 8"x10" hole  and the heater  can screws right on to the stud.  As far as  power is concerned the  750 watt will be fine for a room that size in California and it only draws 3.1 amps (240 volt) but if needed a 1500 watt will fit in the same size rough in can.