Since 2011, DLRs are mandatory for all new cars and small delivery vans in the EU. Trucks and buses followed in August 2012. Vehicles produced before don't have to be retrofitted. Vehicles first homologated before that date do not have to be fitted with DLRs however the ARP4 may have been considered a new homologation because it varied significantly from previous versions and to be secure the DTLs were applied.
Personally I nearly always have the headlights on and I find it a bit strange that the DLR rules only apply to front lights. In our area you often see cars in the tunnels with only front DLRs on.