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If you have the schematic(don't try this with out one) look for a diode called the video detector. If the signal voltage on the output of the detector is about 1V your can inject composite video directly to that point by doing the following...Take a volt meeter and check to make sure that there is not any voltage between the outer (ground) lead of the video cable if it is more than a volt or 2 give up, but if it is less than that connect the outer lead of the video cable to chassis ground (I recommend using a video source for this that you don't mind damaging until you have it working) then measure the voltage between the center pin of the connector and the output of the detector if it is still under 2 volts try connecting it and see if you get video(you may want to insert a small capacitor between them instead of using a direct connection). If not or if black and white are inverted then it either can't be done or you need to use one of the video amp schematics designed for this that if floating around on the net and build a video amp.
None of this is for the non technical and even with all of my experience I tend to shy away from this approach for fear of causing damage and because it is usually easier for me to get acceptable video by fixing bugs in the tuner and IF in of most of my sets. I've only done direct video injection for testing and with equip that has designed to inject a test signal into a TV and not be easily dammaged.
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