Yes essentially. See attached a quick sketch. In my haste I forgot a fuse on the battery feed to the relay - use a fuse! No need for a fuse board or anything elaborate - a neat good inline fuse will work fine - do it as soon as you can after the connection to the battery. You could also use a fused relay - looks like a normal relay but with a standard auto fuse in the top. You can screw these to your battery box. You can copy that circuit for the other heater and just have 2 circuits side by side.
To identify an ign. switched live I would look in the centre of the dash - there are quite a few supplies in there that are only live with the ign. on. I seem to remember about 4-5 - totally disused on my 90 so ideal for this sort of thing. They will be all sorts of colours, there is not a standard colour that represents ign. switched as there are different switched supplies intended to be used for different things.
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