There are 2 problems with with electronic ignition modules retro-fitted to cars that have RVI tacho's:
In the case where the tacho is wired between the coil - terminal and the points, the problem is that people connect the lead that the instructions/wiring diagrams say goes to to the coil - terminal directly to that terminal (which is natural, seeing that's what it tells you to do!). This means that the current flowing from the coil - terminal to the "points" (i.e. the wire which connects the coil to the electronic switcher in the ignition amplifier - the purple one in your case!) never goes through the tacho, so the tacho can't display the engine speed.
However... you say that your purple wire is connected (directly? is there just that wire connected there) to the coil - terminal, and the tacho does display? That tends to suggest your tacho is wired between the coil supply line, OR it's RVC already.
In the case where the tacho is wired between the coil supply line i.e. wired between the 12v supply and the coil + terminal (maybe via the ballast), the problem here is that many people will connect the power supply wire for the ignition amplifier to the coil + terminal. If there is a ballast wire and the supply for the ignition amplifier is piggybacked onto/from the coil + terminal, then that can potentially be a problem.
Apart from the ballast wire causing a volt-drop, which could impair the action of the amplifier - but many have reported that some ignition amplifiers seem to operate satisfactorily even when powered via the ballast - the main problem is that the current flowing through the tacho's "sniffing" terminals is not only the coil-current, but also the amplifier's running current. It's likely that the amplifier's running current going through the tacho will impair it's ability to sense the number of times the coil-LT current fluctuates... therefore impair the ability of the tacho to display the engine revs.