We do offer 4-pole wiring harness kits for your 1998 Volvo V70 and these are all hard-wired types displayed on the linked page. These kits wire in to the leads at the Volvo's taillights. None of these is a plug-and-play option which might be available from a Volvo dealership, albeit at a premium price, IF it is available.
Of the available options I suggest the reliable Tekonsha # C59496 for its combination of reasonable price and high 7.5-amp taillight circuit current capacity which will... view full answer...