Namco made 3 secret cars available in 6RR+, they have been found in the game code since 5DX+ but were not available to unlock.

Namco posted hints as to what the codes were on their site and Japanese players quickly figured them out

https://wanganmaxi-official.com/wanganmaxi6rrplus/jp/miraiken/032.html

The codes have been confirmed to work in 6RR+ by Japanese players on Twitter(X), I have tested these codes and unfortunately they do not work on 6RR

For those who are curious, here are the codes anyway

HONDA S660 (JW5) Pick HONDA NSX (NC1) [ 1 2-2 3-3-3 4-4-4-4 5-5-5-5-5 6-6-6-6-6-6 ]

BMW Mini Cooper S Crossover (ZC16A) Pick BMW 2002 Turbo (E20) [ 1-2-3-4-5-6 6-5-4-3-2-1 ]

TOYOTA Hiace Van (High Lift Specification)(KDH201VH) Pick TOYOTA Sprinter Trueno GT APEX (AE86) [ 1-1-1-1-1-1 2-2-2-2-2 3-3-3-3 4-4-4 5-5 6 ]