hi, if you are wishing for engine pre heat in addition you would require a Thermo Top Evo from Webasto, or if you prefer, a Hydronic S3 D5E CS from Eberspacher. As I am aware there is not a bespokew package available off the shelf from either manufacturer to simply slot into your 1989 Defender, but it should be quite straight-forward to package a heater set for you from our existing stock and accessories. Perhaps send us an email direct to sales@butlertechnik.com and then we could price up a kit build more accurately. Thanks.
PS - interesting that it's 24v, this fact confirms that this kit would not sit at the momenet on anybody's shelf - it would really require a bespoke kit build
Regards