smelly fan holden tr astra?

Almost certainly is the resistor block. The reason that the fan is working only on high is that the resistor block is bypassed when the fan is on high - the burning smell is probably the resistor block dying. The resistor block itself would be mounted somewhere in the fan duct itself (for cooling as it gets hot when in use) and it would clip in from the outside. Can't speak for a TR but for a TS it's behind the glovebox at the top of the duct adjacent the fan itself...

Good Luck.

Anthony.
March 2007