As the previous post indicated, you will have to determine what is leather and what is vinyl on your car. I have a Cab 968 with no backseat so I cannot help you there. This website:
http://www.leatherique.com/ has excellent information on the care and repair of leather (and vinyl) including dyeing. I have used their products for decades and swear by them. On many of my project cars, their products have successfully restored leather that most people said was impossible. I note that they have a distributor in New Zealand--phone # 07 847 8574 or email sales@por15.co.nz
In all likelihood, you should probably redye the entire seat as opposed to trying a touch up.