if you're running the stock heater then the Blue/White is only powering the fan and therefore does not draw too much power.

I noticed that you have another thread that shows an electric demister which would contain its own heating element, this may draw power that a relay could help with.

If you are doing the demister with the standard heater controls (ignoring the original fan) then your diagram just above is fine. Connect the source of Blue/white to 86, yellow to 85 - this will give you a circuit when you flick on the switch (but only one speed)

connect high ampage to 30 via a fuse and from 87 to your heater positive. the relay will click across allowing positive power to the heater when the 86 to 85 is live.

Again, in other words, if battery voltage is applied to the relay coil (terminals 85 and 86 as an electro magnet) terminal 30 will be pulled across and connected to terminal 87. at least this is how it should work

Attach file:

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  relay2.jpg (63.19 KB)