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Many (most?) TV sets of around that time (before switched mode power supplies became common) used a live chassis. I.e, circuit 'ground' is tied directly to one side of the AC mains. You *cannot* add AV inputs to these sets without adding a mains isolating transformer (240v:240V) which is heavy and expensive if big enough for a large colour TV.
Still, if you want to proceed.... :-)
The tuner module (i assume you mean the silver metal box) probably only puts out the video- and sound intermediate frequencies - no good for AV in.
Audio you can easily tap into the 'hot' end of the volume control potentiometer - UNLESS it uses DC controlled volume, (look for a TBA120 chip - a comon one that does the sound detection - you could tap into its output pin)
Video is trickier - does the set use ICs, or is it all discrete transistors? If it uses ICs, check the part numbers until you find the one that does the composite video processing - ONE of its pins will be a suitable input. (if it used the TBA line of chips - I can give u pin numbers to tack into)
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