Calling a function

Having a problem with parsing my declaration and converting into a label.
Heres what i got so far:
Private Function TaxAmount(ByVal TaxAmountDecimal As Decimal) As Decimal

'Declare variable for calculations.
Dim OutputSubtotal_Decimal As Decimal

'Determine tax amount.
Return 0.08D * OutputSubtotal_Decimal

End Function

Private Sub ColorToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ColorToolStripMenuItem.Click
'Declare the variables.

Dim TaxDecimal As Decimal
Dim AmountDue_Decimal As Decimal
Dim CarSalesPrice_Decimal As Decimal
Dim Accessories_Decimal As Decimal
Dim OutputSubtotal_Decimal As Decimal
Dim OutputTotal_Decimal As Decimal
Dim TaxAmount_Decimal As Decimal
Dim TradeInAllowance_Decimal As Decimal

Try
'Convert the Car Sales Price textbox input data.

CarSalesPrice_Decimal = Decimal.Parse(CarSalesPriceTextBox.Text)

Try

'Convert the Trade-In-Allowance textbox input data.

If TradeInAllowanceTextBox.Text = "" Then
TradeInAllowance_Decimal = 0
Else
TradeInAllowance_Decimal = Decimal.Parse(TradeInAllowanceTextBox.Text)
End If

If StereoSystemCheckBox.Checked Then
Accessories_Decimal += STEREOSYSTEM_Decimal
End If

If LeatherInteriorCheckBox.Checked Then
Accessories_Decimal += LEATHERINTERIOR_Decimal
End If

If ComputorNavigationCheckBox.Checked Then
Accessories_Decimal += COMPUTERNAVIGATION_Decimal
End If

'Determine the Amount due.
OutputSubtotal_Decimal = CarSalesPrice_Decimal + Accessories_Decimal
OutputTotal_Decimal = OutputSubtotal_Decimal + TaxDecimal
AmountDue_Decimal = OutputTotal_Decimal - TradeInAllowance_Decimal
TaxDecimal = OutputSubtotal_Decimal + TaxAmount_Decimal

'Display the information.

AmountLabel.Text = AmountDue_Decimal.ToString("c2")
AccessoriesLabel.Text = Accessories_Decimal.ToString
OutputSubTotalLabel.Text = OutputSubtotal_Decimal.ToString
TaxDecimal = Decimal.Parse(TaxLabel.Text)
TaxLabel.Text = TaxAmount(TaxDecimal).ToString
OutputTotalLabel.Text = OutputTotal_Decimal.ToString("C")
TradeInAllowanceTextBox.Text = TradeInAllowance_Decimal.ToString

Catch TradeInAllowanceException As FormatException
MessageBox.Show("Trade-In value must be numeric data.", "Data Entry Error",
MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

With TradeInAllowanceTextBox
.Focus()
.SelectAll()
End With

End Try
I'm trying to calculate the amount due thru the menu item: Calculate and I need to call the tax amount through a function. I hope this makes sense.
I keep getting a message box saying invalid data for the trade-in textbox.