0) Retrieve Fuel Cell Regulator from Klamath (Found in Trapper Town inside the Wrecked Highwayman)
1) The Highwayman we are looking could be found in The Den, however it's broken
2) Talk to Smitty in his Junkyard to find out that the car require Fuel Cell Controller
2.1) Take a Tool in the locker
2.2) Optional - get Wrench in Redding - Mining Camp inside the Morningstar Mine building
3) Head to Gecko - Junkyard and talk to Skeeter to find out that he wants Super Repair Kit for Fuel Cell Controller
3.1) Optional - take a Tool in Skeeters house
4) Head to Vault City
5) Get Wrench by Finding Nixon Doll for Curtis(both near Cassidy's bar)
6) Talk To Valerie in Downtown Vault City (to get the day pass unequip your armor and speak to Wallace)
7) Give Tool (buy one if you still dont have it) and Wrench to Valerie and wait 1 day, then talk to her again to receive Super Repair Kit
8) Return to Gecko - Junkyard and exchange Super Repair Kit for Fuel Cell Controller
9) Return to Smitty in The Den and purchase your car for 2000$
10) Upgrade №1 / Talk to Smitty to install Fuel Cell Regulator (step 0) for 750$ (it doubles the millage)

!!! The car can be charged with either microfusion cells or small energy cells. A full pack of SEC (40) charges it up 20%, a full pack of MFC (50) charges it up 50%. To refuel the car, select the Highwayman, use the "bag" icon from your drop-down menu (hold Right mouse button) and choose the respective energy cells.

11) Head to New Reno - Virgin Street
12) On your foot go to Second Street and then return to Virgin Street
13) The car was stolen! Walk over to where your car was parked and observe the tracks on the ground. You then have the option to follow the tracks to the Chop Shop.
14) Upgrade №2 / Talk to T-Ray and pay $1000 for the car ($750 or $500 if you pass a Barter check). For $500 ($300 with decent Barter) extra, T-Ray will upgrade the trunk and the speed
15) Upgrade №3 / Head to NCR Bazaar and talk to Ratch for $1000, he can install a blower, which increases speed, into the Highwayman. The process will take six hours.

Bonus!
Talk to T-Ray after completion of the game ;)