FIRST YOU NEED TO HAVE RED DEAD RADIO MOD FOR THE SOUNDTRACK TO WORK !!!
red dead radio: https: https://www.nexusmods.com/reddeadredemption2/mods/529
rdr1 soundtrack: https: //www.nexusmods.com/reddeadredemption2/mods/666
rdr1 undead nightmare soundtrack: https: //www.nexusmods.com/reddeadredemption2/mods/672
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You just need to put the songs in the Radio 1, Radio 2, Radio3 folders they are located in Red Dead Redemption 2\Red Dead Radio
To download the Red dead radio mod just use the link above.
When you install you just need to press Q + R to open the Radio inside the game and select the radio and soundtrack you want to use.
Remember the songs will not play automatically you need to do it yourself.
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if you are having problems installing mods you can find a tutorial here: https://www.youtube.com/watch?v=uISWfVHPhlE&t=19s