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I'd need to see the schematic to be able to comment on the exact connection...
Generally speaking you want to run the TV and only the TV on the isolation transformer*, Ground the cable shield to chassis, and connect the center of the coax to a point after the detector with low or no DC voltage relative to chassis ground tube grids are good. If all points have DC voltage then use cap .1 uF or larger (you can go as high as you like) in series with the coax center and injection point. Choosing an injection point can be easy or impossible....You see each tube (video and IF) that the signal goes through inverts the video so if you inject at the grid of the first video amp you could get negative video (black and white reversed) , and injecting at the plate could give you positive video or vice-versa....You have to choose an injection point that gives you positive video, then you have to choose one with a comparable designed signal level....If you inject your signal at a point that wants more signal you may not get enough contrast, if you inject at a point that wants too little signal you may get too much contrast. Some sets don't an injection point that is the right polarity AND is easily able to accept composite video signal levels....When that happens you have to make a additional video amp circuit or go back to RF.
It is advisable to disable the IF system for injection (unless you get no audio or video noise with no RF signal input)....On a series string set the best way may be to find a dud last video IF tube with a good heater and snip off or bend the plate pin (if the detector diode is easy to get at hat would work too)...If using audio injection you may want to do that at the audio detector too.
* I've seen isolation transformers loose isolation from primary to secondary shorts so it is wise to check for voltage between a leg of the primary and a leg of the secondary with the transformer powered (should be 0 volts).
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