Test it with the trunk opened but then lock the latch using a screwdriver, then see if the latch unlocks with the remote as it should (might take 2 tries if it's the dual stage type where one click unlocks the drivers door only and the second click unlocks the others). That way, it'll have nothing to do with the weight of the spoiler. If it does then unlock as it should (like if the dealer is right), then I'd suggest replacing your trunk shocks with the HD type made by Volvo for cars with spoilers (not sure about C70s though?). When adding the OEM Volvo rear spoiler to an S70 (never installed on on a C70), Volvo then supplies HD replacement trunk shocks to handle the added weight included with the rear spoiler kit.