You don't need "variable" variable names. Just rename the columns before you write out to CSV.
e.g.
columns = ["sedan", "wagon", "convertible"]
new_columns = [f"{c}_dom" for c in columns]
Here's a pandas version:
import pandas as pd
from io import StringIO # not required if reading csv from file
# Dummy csv to demo, can also read from csv file
csv = """dealer,car,sedan,wagon,convertible, junk
A,Ford,Red,0,Green, None
A,GMC,Blue,Red,0, None
B,BMW,Green,0,Red, None
B,Ford,0,Blue,Red, None
C,BMW,0,Green,0, None
C,GMC,Red,Blue,Red, None
D,GMC,Red,Blue,Blue, None
D,GMC,Red,Blue,0, None
"""
columns = ["sedan", "wagon", "convertible"] # ignore the junk etc, columns that aren't of interest
dom_columns = {c: f"{c}_dom" for c in columns}
df = pd.read_csv(StringIO(csv)) # or df = pd.read_csv(r"path/to/csv")
dom_df = df[columns].mode().rename(columns=dom_columns)
dom_df.to_csv("dom.csv", index=False)