A replacement vehicular lamp assembly (10) configured for insertion into a vehicle chassis-mounted lamp housing (110) originally containing an incandescent lamp (118) installed in an aperture (102) in a chassis-mounted reflector (116). Lamp assembly (10) has a housing (12) supporting LED(s) and displaceably mounted reflector segments (40) moveable between a closed configuration facilitating installation through the aperture (102) and an outwardly open configuration for function within the lamp housing (110) to form a first reflector (38). A biasing mechanism (25) biases the reflector segments (40) to the open configuration to deploy the first reflector (38) overlying the chassis-mounted reflector (116) so that light from the LED is received on the first reflector (38) defined by the reflector segments (40).