Globetrotter 1959 - running lights junction box

[Vince_59GT]

Hi,
i need to change the wiring between the junction box to the plug to towing car on my 59 globetrotter. Do someone know where the junction box is located and is there an easy access to it (i didn't see any access....)? or do i need to cut a hole on the belly pan ?

Thanks for your advice !