Which version are you building? If it is the Early version (kit # 02306), it is not used. If it is the Late version (kit# 02319), it goes on top of the gauge cluster for the brakes, see below.
By the way, the kits are the same and either version can be built from either kit. Just follow the instruction diagram above and add the guage cluster (B29, B30, PE-6) and the extension to the hydraulic resevoir (B35) to build the later version. Leave these parts off for the early version.